Faraday's Law states that the movement of a conductor across a magnetic field produces a current in the conductor. In M.H.D. power generation the conductor is an ionized gas moving across the magnetic field. Ionized gas is produced by extremely high temperatures (plasmas) or lower temperatures combined with alkali metal seedings. If the flow of ionized gas is perpendicular to the applied magnetic field, an electric field is generated perpendicular to both magnetic field and ionized gas flow directions. Electricity can be extracted from this energy transfer either by placing electrodes in the electric field and drawing current from them, or by inducing current in a search coil by strategically placing the coil in the vicinity of the ionized gas in motion.
The problems of containing high temperature plasmas by the pinch effect of magnetic fields, and also the extreme corrosivity of the alkali metal seeding, have not yet been resolved. A third method for ionizing gases (and the one preferred here) is that of using high speed shock waves to induce ionization. In this way the very high temperature of the plasma and the corrosion of the alkali seed are avoided.
Experiments with linear shock tubes have shown that it is advantageous to produce an ionized gas which is homogeneous, free of currents, and which can be changed in composition and degree of ionization. Electromagnetic shock tubes can result in high Mach numbers, but the shock-induced ionization is often unstable, inhomogeneous and perturbed by eddy currents. Membrane shock tubes do not provide for reaching as high a Mach number and therefore do not produce as high a degree of ionization. Efforts made to increase sonic speeds in the driver gas of the membrane shock tube led to the use of combustible mixtures of hydrogen and oxygen as driver gas. Higher Mach numbers were obtained than was possible with cold hydrogen at the same pressure ratio. The continuous breaking of membranes from the energy of a hydrogen/oxygen explosive combustion is expensive and difficult from an engineering point of view. This is due to the fact that moveable parts are involved, since fresh membranes must be presented to the explosion.